Phototropism

Photo by: Trevor Slauenwhite

Phototropism (pronounced foe-TA-tro-piz-em) is the growth of a plant in
the direction of its light source. Plants are very sensitive to their
environment and have evolved many forms of "tropisms" in
order to ensure their survival. A tropism is the growth of a plant as a
response to a stimulus, and phototropism occurs when a plant responds to
light by bending in the direction of the light. Although plant
physiologists (scientists who study how the processes of a plant actually
work) know that this growth is caused by a plant hormone, they still do
not fully understand exactly how it works.

Bending toward the light

Most of us at some time have noticed a houseplant on a windowsill that
seems to have all of its thin stems leaning in the same direction, as if
it were trying to press itself against the glass. Picking it up and
turning the entire pot in the opposite direction so that the plant is
pointing away from the window will only result, about eight hours later,
in the plant having reversed itself and going about its business of
pointing its leaves toward the window again. This is not because plants
especially like

Plants respond to the direction and amount of light they receive. The
seedlings on the left grew toward the light it received on only one
side. The plant in the center received no light. The plant on the
right was grown in normal, all-around light.
(Reproduced by permission of

Photo Researchers, Inc.

)

windows but rather because light is essential to their survival, and they
have developed ways of making sure they get all they need.

We know then that it is the light coming through the window that the
plants are striving to get closer to, but how is a plant, which is rooted
in soil, able to "move" toward the light? Actually, the
plant does not so much move toward the light source as it grows in that
direction. As already noted, this growth of a plant that occurs as a
response to a stimulus is called a tropism. There are several forms of
tropisms, such as gravitropism or geotropism, in which a plant reacts to
the force of gravity; hydrotropism, in which the presence of water causes
a response; galvanotropism, in which a plant reacts to a direct electrical
current; thigmotropism, in which a plant responds to being touched or some
form of contact; and chemotropism, in which a plant reacts to a chemical
stimulus. Since the prefix "photo" refers to light,
phototropism involves a plant responding to light. In all of these
tropisms, the plant's response involves some form of growth.
Finally, all tropisms are either positive or negative, although these
words are not always used. So when a plant's leaves grow toward the
light (stimulus), it is technically called positive phototropism. When its
roots normally grow away from the light, it is called negative
phototropism.

Words to Know

Auxin:
Any of various hormones or similar synthetic substances that regulate
the growth and development of plants.

Photosynthesis:
Chemical process by which plants containing chlorophyll use sunlight to
manufacture their own food by converting carbon dioxide and water to
carbohydrates, releasing oxygen as a by-product.

Tropism:
The growth or movement of a plant toward or away from a stimulus.

How phototropism works

It is known that as long ago as 1809, Swiss botanist Augustin Pyrame de
Candolle (1778–1841) observed the growth of a plant toward the
light and stated that it was caused by an unequal growth on only one part
of the plant. However, he could not understand how this was happening.
Some seventy years later, English naturalist Charles Darwin
(1809–1882) began to grow canary grass in order to feed the birds
he kept, and he eventually discovered that it was the tips of the
sprouting seedlings that were influenced by the direction of their light
source. He and his son Francis learned this when they covered the tips of
some seedlings and found that they did not move toward the light. When
only the seedlings' stems were covered, however, they still moved
toward the light.

It was not until the 1920s that Dutch botanist Frits W. Went
(1903–1990) proved the connection between phototropism and a plant
hormone called auxin. Went discovered that plants manufacture a growth
stimulant (which he named auxin) in their tips, which they then send to
other cells in the plant. In phototropism, however, this growth hormone is
distributed unevenly when the light source comes from only one direction.
Specifically, more auxin flows down the dark side, meaning that it grows
faster than the exposed side of the plant. This unequal or one-sided
growth (also called differential growth) brings about the curving or
bending of the plant toward the light source. Went named this growth
hormone after the Greek word
auxein,
which means "to increase." Although it was isolated and
named, auxin was not understood chemically until twenty years later when
it was finally identified chemically as indole-3-acetic acid.

Plants can react and adjust

Understanding what plant tropism is and, specifically, what happens during
phototropism makes us realize that plants, as living things, necessarily
demonstrate the several characteristics of life. Specifically, this
includes growth, response to stimuli, and adaptation. It is because of its
hormones that a plant's stem always grows upwards and its roots
always grown downward. Since plants must make their own food to survive
(by changing light energy into chemical energy—a process called
photosynthesis), the ability to capture as much of this light energy as
possible is crucial to its survival. Thus, plants have developed a
chemical response to light or the lack of it that causes their stems to
bend toward the stronger light.

Today, we know that a certain minimal amount of light (whether natural or
artificial) has to be present for the plant to react chemically. This is
called its threshold value. Despite our understanding of the basic stages
and phases of phototropism, we are only now beginning to obtain the most
basic knowledge of what goes on at the genetic and molecular level. We do
realize however that plants are living, sensitive things that can adjust
to their environment and actually seek out the light they need if they are
not getting enough.

Great information and unlike other websites, i actually understand what i'm reading but it would be great if you could describe the other types of tropism a bit more. Thank you for all the help and great work!

This was the most helpful website i absoluetly loved iT! The pictures and information were the bomb!! Wikepedia is way to complicated and this was understandable I will recomend this website to anyone for a science project becuase it really helped!!! Thanks a million

Omg this really helped me and my science project!!! Good information! I'm doing a project on how plants respond to light by being put at different points in a room, but my farthest plant away looks healthy, but I was expecring it to be the one to most likely lean the most towards the window. Any tips on the subject?

This was a fantastic website that really helped me with my science fair project. I am studying how plants navigate objects to find sunlight. This website helped me to get a better understanding of phototropism. Thanks!